PROGESTERONE SUPPRESSION OF GLUTAMIC-ACID DECARBOXYLASE (GAD(67)) MESSENGER-RNA LEVELS IN THE PREOPTIC AREA - CORRELATION TO THE LUTEINIZING-HORMONE SURGE

The progesterone-induced LH surge in the estrogen-primed rat is though t to be mediated through interneurons since LHRH neurons reportedly la ck steroid receptors. Along these lines, glutamate and gamma-aminobuty ric acid (GABA) neurons are considered to be two of the major interneu rons in the hypothalamus involved in the control of LHRH and LH secret ion. Glutamate is a major excitatory amino acid neurotransmitter while GABA is a major inhibitory amino acid neurotransmitter in the control of LH secretion. Glutamate is converted in the brain to GABA by the r ate-limiting enzyme glutamic acid decarboxylase (GAD). Regulation of G AD by steroid hormones would therefore appear to be a logical mechanis m for the control of LHRH and LH secretion based on the changes in glu tamate and GABA production which would result from such regulation. Th erefore, the purpose of the present study was to determine whether the steroid hormone, progesterone, regulates the GAD enzyme in the hypoth alamus at a time when it induces an LH surge. To accomplish this aim, northern and dot blot analysis were used to measure GAD(65) and GAD(67 ) mRNA levels in the preoptic area (POA) and medial basal hypothalamus (MBH) in ovariectomized (OVX) immature rats, OVX rats primed with 2 m u g of estradiol (E(2)) for 2 days, and OVX rats primed with 2 mu g of E(2) for 2 days and administered progesterone (1 mg/kg, 09.00 h) to i nduce an LH surge. The results of the study show that GAD(67) mRNA lev els were suppressed by progesterone in the POA at 12.00 h (at the star t of the surge) (24.4% suppression vs. E(2) only; p < 0.01) in the fir st experiment. This suppression was confirmed in a second experiment a s progesterone suppressed GAD(67) mRNA levels in the POA at 12.00 h (3 1.8% vs. E(2) only; p < 0.05) and also at 14.00 h (40% vs. E(2) only; p < 0.05) which correlates to the peak of the LH surge. The changes in GAD(67) by progesterone were restricted to the POA, as GAD(67) mRNA l evels in the MBH were unaffected by the steroid treatment. Furthermore , the effect of progesterone on POA GAD(67) mRNA levels was specific a s it was blocked by prior treatment with the antiprogestin, RU486 (400 mu g), In the MBH, both GAD(67) and GAD(65) mRNA levels were increase d by estradiol treatment at 16.00 h and this increase was prevented by progesterone administration in the presence of estrogen priming. In c onclusion, these studies demonstrate that the mRNA for the key enzyme responsible for the conversion of the major excitatory amino acid neur otransmitter in the hypothalamus (glutamate) to the major inhibitory a mino acid neurotransmitter (GABA), is regulated by the steroid hormone progesterone. Progesterone suppression of GAD(67) mRNA levels in the POA, where LHRH neurons reside, and at a time immediately preceding an d during the LH surge, could provide a mechanism for greater stimulati on of LHRH neurons through enhanced excitatory glutamate production co ncomitant with decreased inhibitory GABA production in the POA.